Final answer:
A secondary mode of regulation for tryptophan, known as attenuation, provides precise control over the trp operon's expression in E. coli on top of the primary repressor-operator regulation. It allows for fine-tuned responses based on the cellular levels of tryptophan, preventing the waste of resources.
Step-by-step explanation:
The need for a secondary mode of regulation for Trp (tryptophan) is important because cells must adapt to varying internal and external environmental conditions. This secondary regulation, known as attenuation, is a mechanism that controls the expression of the trp operon in E. coli. Attenuation relies on the formation of secondary stem-loop structures within the 5' end of an mRNA. The leader sequence called trpL has four stretches of RNA that can base pair in different ways leading to the formation of a terminator stem-loop, which results in transcription termination, or an antiterminator stem-loop, which allows RNA polymerase to transcribe structural genes.
Thus, in addition to the primary repressor-operator regulation, attenuation provides a rapid, finely-tuned response based on the presence of tryptophan. If tryptophan is abundant, it binds to the trp repressor, changing its shape and enabling it to bind to the trp operator to block transcription. When tryptophan is scarce, the operon is active, allowing for its synthesis.
This dual regulatory system ensures precise control of tryptophan synthesis, preventing waste of resources through feedback inhibition and attenuation, both crucial processes in gene regulation.